1. Field of the Invention
The invention relates to fixing roller devices for photocopiers comprising a pressure roller and an internally heated heating roller as the fixing roller for fixing a toner image on copying material.
2. Background Art
A fixing roller device is the component of a photocopier which fixes toner, usually a particulate plastic material, which has been electrostatically deposited upon a copy medium, most often paper. Such a fixing roller device has two cylindrical rollers in lengthwise contact. One roller is internally heated, and is called the heating roller. The other presses against the heating roller and is called the pressure roller. The paper bearing the unfixed toner passes between the rollers, and the combination of heat and pressure causes the toner to fuse and adhere to the paper. The surface temperature of the heating roller is ideally neither so low that fixing is incomplete and the toner easily wiped off, nor so high that toner is transferred from the paper to the roller, producing a so-called offset effect, in which part of the toner image is subsequently transferred onto other areas of the copying paper.
It was believed in the past that the surface temperature distribution along the axial (lengthwise) direction should ideally be as uniform as possible. Nevertheless, a typical prior art fixing roller device includes a heating roller having an electrical heating element arranged in its interior, parallel to the roller axis. This causes the surface of the heating roller to have an uneven surface temperature distribution in the axial direction. A temperature sensor is typically located opposite the surface of the heating roller and is connected to a control system which supplies current to the heating element.
A heating fixing roller device of this type is shown in German Pat. No. 2,949,996. In this device, the heating element is an infrared radiator which exhibits a higher radiation density at its ends than in its middle. When it is switched on, the uneven radiation density results in an uneven surface temperature distribution in the axial direction of the fixing roller having a minimum in the middle and at each of the two axial ends, and a maximum between each end minimum and the middle minimum. The sole temperature sensor is located near the surface of the fixing roller which is at the maximum temperature.
As stated above, in this fixing roller device there is no temperature distribution maximum in the middle; instead the maxima occur between the two ends and the middle of the fixing roller. Even with such an arrangement an undesirable overshooting of the intended temperature values will occur at the temperature distribution maxima. This effect is compensated, however, by the presence of the zones of lower temperature in the middle and at the two ends of the heating roller, and any tendency of the temperature distribution to even out and reach a uniform equilibrium temperature. The overshooting of the temperatures results from the fact that while the temperature sensor, upon detecting the preset desired temperature, switches off the current supply to the heating element, the fixing roller has by that time already stored a substantial amount of heat and the "thermal inertia" of the system produces temperatures above the preset value.
German Offenlegungsschrift No. 3,224,239 discloses a roller arrangement for thermal fixing, comprising a heating roller and a pressure roller, between which moves paper carrying a toner image. The heating roller contains a heating element by means of which the surface temperature of the heating roller is kept at a predetermined value. On switching on the copying apparatus, the heating roller and the pressure roller rotate before the surface temperature of the heating roller has reached the predetermined temperature. As soon as this temperature has been reached the motor which causes the heating roller and the pressure roller to rotate is switched off. The heating element in the heating roller is controlled by means of a temperature sensor, which is located at a position to measure the surface temperature of the heating roller.
U.S. Pat. No. 4,323,959 discloses a toner image fixing apparatus comprising a heating roller and a pressure roller, in which a temperature sensor measures the surface temperature of the heating roller. The magnitude of the force with which the pressure roller presses against the heating roller is regulated according to the measured surface temperature. This is supposed to achieve uniform quality of the toner image on a sheet transported between the two rollers by controlling two parameters which most influence the fixing process, the temperature and the contact pressure. If, for example, the surface temperature increases, the contact pressure is reduced, or the surface temperature is descreased while the contact pressure increases. The interrelation between these two parameters is regulated in accordance with a predetermined surface temperature/contact pressure relation.
The roller fixing station disclosed in European patent application No. 0,017,092 is equipped with a pair of rollers, one of which has outward-tapering end portions. The roller has an outer sleeve which is centrally mounted on an axle. The main parts of the inner surface of the sleeve have stepped portions with increasing diameter at each tapering end of the sleeve. In the tapering end portions of the sleeve, screwable plugs are fixed to parts of the axle. The main body of each plug has a smaller diameter than the surface portion of the inner sleeve. The plugs are rotatable on the axle and move linearly from a retracted position, in which there is clearance between the plugs and the sleeve of the roller, into an engagement position, in which the end flanges of the plugs are in engagement with the stepped portions of the sleeve. In the retracted position the end portions of the sleeve are not supported mechanically by the flanges of the plugs so that the pressure of the counter-roller presses the tapering end portions of the sleeve against the plugs. The degree of taper and the magnitude of the gaps between the plugs and the sleeve are matched to one another so that the behavior of the roller corresponds to that of an essentially even, i.e., non-tapering, roller.
The tapering construction of the roller is retained if the end plugs are screwed inwardly, in the axial direction, and this prevents creasing of the copying paper which usually occurs in the fixing station if the moisture content is high.
If the end plugs are screwed outwardly, in the axial direction, the roller operates like an essentially even cylindrical roller which does not have a taper. Because of the contact pressure of the counter-roller the tapering configuration of the roller is flattened off. The use of this roller under these conditions prevents the so-called smudging effect of a copy under dry conditions, i.e., at very low atmospheric humidity in the fixing station. The plugs can be adjusted manually when the two rollers are separated from one another or can be adjusted by means of a motor which is controlled by a humidity sensor.
The taper achieves a higher cicumferential speed near the edge of the roller so that a sheet of paper passing through the gap between the rollers is subjected to a peripheral speed along its edges which is higher than the speed in the middle. The result of this is that the copying paper stretches and does not crease even at high relative humidity in the fixing station.
Under very dry conditions the copying paper tends to crinkle or form small corrugations so that when the copying paper enters the fixing device it contacts the fixing roller too early, causing smudging of the image.
Thus, in known fixing devices, a very uniform temperature distribution over the length of the fixing roller, with compensation for the temperature drop near the roller ends, is sought in order to maintain constant copy quality. Alternatively, the contact pressure between the fixing roller and the pressure roller is regulated as a predetermined function of the measured temperature. It is also known to use a roller which tapers outwardly in the end portions which flattens under the contact pressure of an adjacent roller, thereby giving uniform contact pressure over the length of the roller and thus avoid creasing.